My mouse is moving very hardly and slow, just at the moment i am rendering an image
Go to the task manager and begin to render something. Right click on the blender task that is taking the most processing power and press "go to details". When you have done that you should see a lot of programs and one of them will be marked(blender will be marked). Right click on it and go to 'set priority'  and set the priority to something low.

Now its fixed(hopefully) but your renders might take longer. Though I'm not sure if it's a noticeable amount slower.